The mixed-function oxidase, cytochrome P-450, is a major liver protein consisting of several distinct forms which can be induced by different environmental conditions. The long-term goal of this laboratory is to determine the molecular basis of this induction process. In this study it is proposed to determine whether the major effect of phenobarbital, a cytochrome P-450 inducing agent, is exerted at the transcriptional or translation level. Rat liver RNA isolated at different time points following phenobarbital administration will be translated in a cell free system to determine the profile of cytochrome P-450 synthesis following initiation of induction. Newly synthesized cytochrome P-450 will be assayed using an antibody specific for the phenobarbital-induced form of this protein. This antibody will also be used in the isolation of polysomes specific for cytochrome P-450 synthesis. The cytochrome P-450 specific mRNA extracted from these polysomes will be used as a template for synthesis of a radioactive complementary DNA (cDNA) copy and this probe will be used in hybridization experiments to assay cytochrome P-450 specific mRNA levels as a function of time following phenobarbital administration. Comparison of the results obtained from these two studies will provide the answer as to the level of the major inductive effect of phenobarbital. Cross-hybridization experiments will be carried out with above cDNA probe and RNA isolated from animals induced with 3-methylcholanthrene to compare this inducing agent with phenobarbital. 3-Methylcholanthrene is known to induce a form of cytochrome P-450 which is distinct from that induced by phenobarbital. These studies are designed to provide the basis on which the molecular mechanism of xenobiotic induction of cytochrome P-450 can be determined.